1. Field of the Invention
This invention relates to a method of controlling a control system, and more particularly, to a method of controlling a control system required to have a good robustness to disturbances, such as a control system for a motor.
2. Description of the Prior Art
In conventional control systems, a PI controller (Proportional-Integral controller) has been widely used in order to improve the robustness to disturbances, or to supress the influences of disturbances. Recently, the PI controller can provide a digital control using a microprocessor. For example, a digital PI controller using a microprocessor for controlling the speed of a DC motor is disclosed in "Digital Control Systems", chapter 14, section 14.4, pp 689-694, written by Benjamin C. Kou and published as Holt-Saunders International Editions.
The PI controller, whether analog or digital, can improve the robustness to disturbances more at a lower frequency. However, the recent requirement to improve the robustness to disturbances has become more and more severe in some applications. For example, a control system for controlling the speed of a capstan motor in a video tape recorder requires a highly improved robustness to torque disturbances, because the size and the inertia of the capstan motor have been greatly minimized in recent years.
A new control system for improving robustness to disturbances was submitted in my previous U.S. patent application Ser. No. 917,498 filed Oct. 10, 1986. This control system has an excellent robustness to disturbances, but it requires a large RAM (random access memory) capacity which make the system expensive.
Recent integrated-circuit technology can make a small RAM capacity on a single chip microprocessor. So, it is possible to make the control system cheaper and simpler if the RAM size used is small.